Ajay Kumar at 05:19, 7 August 2023

2023-08-07T05:19:55Z

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{{short description|Orbit of an object around the Moon}}
{{For|the orbit of the Moon around the Earth|Orbit of the Moon}}
[[File:AS8-13-2329.jpg|thumb|right|250px|The [[Moon]] from lunar orbit, with planet [[Earth]] rising over the [[horizon]], taken on the [[Apollo 8]] mission by astronaut [[Bill Anders|William Anders]] on December 24, 1968.]]

In [[astronomy]], '''lunar orbit''' (also known as a '''selenocentric orbit''') is the [[orbit]] of an object around the [[Moon]].

As used in the [[space program]], this refers not to the [[orbit of the Moon]] about the [[Earth]], but to orbits by [[spacecraft]] around the Moon. The [[altitude]] at [[apoapsis]] (point farthest from the [[Center of mass|center of attraction]]) for a lunar orbit is known as '''apolune''', '''apocynthion''', or '''aposelene''', while the [[periapsis]] (point closest to the center of attraction) is known as '''perilune''', '''pericynthion''', or '''periselene''', from names or epithets of the [[moon goddess]].

'''Lunar orbit insertion''' ('''LOI''') is the adjustment to achieve lunar orbit, as undertaken by [[Apollo program|Apollo]] spacecraft for example.<ref>{{cite book| year=2008 | chapter=Entering lunar orbit: the LOI manoeuvre | last=Woods | first=W.D. | title=How Apollo Flew to the Moon | publisher=Springer Praxis Books | series=Space Exploration | pages=189–210 | doi=10.1007/978-0-387-74066-9_8 | isbn=978-0-387-71675-6 }}</ref>

'''Low lunar orbit''' ('''LLO''') are orbits below {{convert|100|km|mi|abbr=on}} altitude. They have a period of about 2 hours.<ref name="nasa20061106" /> They are of particular interest in exploration of the Moon, but suffer from gravitational [[perturbation (astronomy)|perturbation]] effects that make most unstable, and leave only a few [[orbital inclination]]s possible for indefinite ''[[frozen orbit]]s'', useful for long-term stays in LLO.<ref name=nasa20061106/>

[[File:Delta V Earth Moon Mars.png|thumb|Depiction of [[delta-v]] to Lunar orbits.]]

==Robotic spacecraft==
{{expand list|date=October 2020}}
[[File:Earth rising behind the Moon.jpg|thumb|[[Orion capsule]] of [[Artemis 1]] above the Moon]]
The [[Soviet Union]] sent the first spacecraft to the vicinity of the Moon, the robotic vehicle [[Luna 1]], on January 4, 1959.<ref name="mark">{{cite web
|last = Wade
|first = Mark
|title = Luna
|publisher = [[Encyclopedia Astronautica]]
|url = http://www.astronautix.com/project/luna.htm
|access-date = 2007-02-17
|url-status = dead
|archive-url = https://web.archive.org/web/20120111203409/http://www.astronautix.com/project/luna.htm
|archive-date = 2012-01-11
}}</ref> It passed within {{convert|6,000|km|nmi mi}} of the Moon's surface, but did not achieve lunar orbit.<ref name="mark"/> [[Luna 3]], launched on October 4, 1959, was the first robotic spacecraft to complete a [[Circumlunar trajectory|circumlunar]] [[free return trajectory]], still not a lunar orbit, but a figure-8 trajectory which swung around the [[far side of the Moon]] and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface.<ref name="mark"/>

The [[Luna 10]] became the first spacecraft to actually orbit the [[Moon]] in April 1966.<ref name="beyers">{{cite web
| last = Byers
| first = Bruce K.
| title = APPENDIX C [367-373] RECORD OF UNMANNED LUNAR PROBES, 1958-1968: Soviet Union
| work = DESTINATION MOON: A History of the Lunar Orbiter Program
| publisher = National Aeronautics and Space Administration
| date = 1976-12-14
| url = http://www.hq.nasa.gov/office/pao/History/TM-3487/app-c.htm#soviet
| access-date = 2007-02-17
| archive-date = 2021-01-26
| archive-url = https://web.archive.org/web/20210126065856/https://www.hq.nasa.gov/office/pao/History/TM-3487/app-c.htm#soviet
| url-status = dead
}}</ref> It studied [[micrometeoroid]] flux, and lunar environment until May 30, 1966.<ref name="beyers"/> A follow-on mission, [[Luna 11]], was launched on August 24, 1966, and studied lunar gravitational anomalies, radiation and solar wind measurements.

The first United States spacecraft to orbit the Moon was [[Lunar Orbiter 1]] on August 14, 1966.<ref name="orbiter">{{cite web
| last = Wade
| first = Mark
| title = Lunar Orbiter
| publisher = Encyclopedia Astronautica
| url = http://www.astronautix.com/craft/lunbiter.htm
| archive-url = https://web.archive.org/web/20020821173650/http://astronautix.com/craft/lunbiter.htm
| url-status = dead
| archive-date = August 21, 2002
| access-date = 2007-02-17}}</ref> The first orbit was an [[elliptical orbit]], with an [[apolune]] of {{convert|1,008|nmi|km mi}} and a [[perilune]] of {{convert|102.1|nmi|km mi}}.<ref>{{cite web
| last = Byers
| first = Bruce K.
| title = CHAPTER IX: MISSIONS I, II, III: APOLLO SITE SEARCH AND VERIFICATION, The First Launch
| work = DESTINATION MOON: A History of the Lunar Orbiter Program
| publisher = National Aeronautics and Space Administration
| date = 1976-12-14
| url = http://www.hq.nasa.gov/office/pao/History/TM-3487/ch9-2.htm
| access-date = 2007-02-17
| archive-date = 2020-09-27
| archive-url = https://web.archive.org/web/20200927145230/https://www.hq.nasa.gov/office/pao/History/TM-3487/ch9-2.htm
| url-status = dead
}}</ref> Then the orbit was circularized at around {{convert|170|nmi|km mi}} to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitable [[Apollo program]] landing sites.<ref name="orbiter"/>

The most recent was the [[Lunar Atmosphere and Dust Environment Explorer]] (LADEE), which became a ballistic impact experiment in 2014.

==Crewed spacecraft==
The [[Apollo program]]'s [[Apollo Command/Service Module|Command/Service Module]] (CSM) remained in a lunar parking orbit while the [[Apollo Lunar Module|Lunar Module]] (LM) landed.
The combined CSM/LM would first enter an elliptical orbit, nominally {{convert|170|nmi|km mi}} by {{convert|60|nmi|km mi}}, which was then changed to a circular parking orbit of about {{convert|60|nmi|km mi}}. Orbital periods vary according to the sum of [[apoapsis]] and [[periapsis]], and for the CSM were about two hours. The LM began its landing sequence with a Descent Orbit Insertion (DOI) burn to lower their [[periapsis]] to about {{convert|50,000|ft|km nmi}}, chosen to avoid hitting [[List of mountains on the Moon|lunar mountains]] reaching heights of {{convert|20,000|ft|km nmi}}. After the second landing mission, the procedure was changed on [[Apollo 14]] to save more of the LM fuel for its powered descent, by using the CSM's fuel to perform the DOI burn, and later raising its [[periapsis]] back to a circular orbit after the LM had made its landing.<ref name="a11fj">{{cite web
| last = Jones
| first = Eric M.
| title = The First Lunar Landing
| work = Apollo 11 Lunar Surface Journal
| publisher = National Aeronautics and Space Administration
| date = 1976-12-14
| url = https://www.hq.nasa.gov/alsj/a11/a11.landing.html
| access-date = 2014-11-09}}</ref>

==Perturbation effects==
{{further|Lunar mascons}}
Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of [[mass concentration (astronomy)|mass concentration]]s (dubbed '''mascons''') beneath the lunar surface caused by large impacting bodies at some remote time in the past.<ref name=nasa20061106/><ref>{{Cite journal|last1=Konopliv|first1=A. S.|last2=Asmar|first2=S. W.|last3=Carranza|first3=E.|last4=Sjogren|first4=W. L.|last5=Yuan|first5=D. N.|date=2001-03-01|title=Recent Gravity Models as a Result of the Lunar Prospector Mission|journal=Icarus|volume=150|issue=1|pages=1–18|doi=10.1006/icar.2000.6573|bibcode=2001Icar..150....1K|issn=0019-1035}}</ref>
These anomalies are of sufficient magnitude to cause a lunar orbit to change significantly over the course of several days. They can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent.<ref name="nasa20061106" />
The [[Apollo 11]] first manned landing mission employed the first attempt to correct for the perturbation effect (the frozen orbits were not known at that time). The parking orbit was "circularized" at {{convert|66|nmi|km mi}} by {{convert|54|nmi|km mi}}, which was expected to become the nominal circular {{convert|60|nmi|km mi}} when the LM made its return [[Space rendezvous|rendezvous]] with the CSM. But the effect was overestimated by a factor of two; at rendezvous, the orbit was calculated to be {{convert|63.2|nmi|km mi}} by {{convert|56.8|nmi|km mi}}.
<ref name="Apollo 11 Mission Report">{{Cite web|url=https://history.nasa.gov/alsj/a11/A11_MissionReport.pdf|title=Apollo 11 Mission Report|publisher=[[NASA]]|pages = 4–3 to 4–4}}</ref>

Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 of "[[frozen orbit]]s" occurring at four [[orbital inclination]]s: 27°, 50°, 76°, and 86°, in which a spacecraft can stay in a low orbit indefinitely.<ref name=nasa20061106>
{{cite web |title=Bizarre Lunar Orbits|url=https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/ |work=NASA Science: Science News |date=2006-11-06 |publisher=NASA |access-date=2012-12-09 |quote=''Lunar mascons make most low lunar orbits unstable ... As a satellite passes 50 or 60 miles overhead, the mascons pull it forward, back, left, right, or down, the exact direction and magnitude of the tugging depends on the satellite's trajectory. Absent any periodic boosts from onboard rockets to correct the orbit, most satellites released into low lunar orbits (under about 60 miles or 100 km) will eventually crash into the Moon. ... [There are] a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27°, 50°, 76°, and 86° — the last one being nearly over the lunar poles. The orbit of the relatively long-lived [[Apollo 15]] subsatellite [[PFS-1]] had an inclination of 28°, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11°.''}}</ref> The [[Apollo 15]] subsatellite [[PFS-1]] and the [[Apollo 16]] subsatellite [[PFS-2]], both small satellites released from the Apollo [[Apollo Command/Service Module#Service Module (SM)|Service Module]], contributed to this discovery. PFS-1 ended up in a long-lasting orbit, at 28° [[Orbital inclination|inclination]], and successfully completed its mission after one and a half years. PFS-2 was placed in a particularly unstable orbital inclination of 11°, and lasted only 35 days in orbit before crashing into the lunar surface.<ref name=nasa20061106/>

==See also==
*[[Cislunar space]]
*[[List of orbits]]
*[[Orbital mechanics]]
*[[Distant retrograde orbit]]
*[[Near-rectilinear halo orbit]]

==References==
{{reflist}}

{{orbits}}

[[Category:Moon]]
[[Category:Orbit of the Moon]]

Lunar orbit - Revision history

Ajay Kumar at 05:19, 7 August 2023